1. Field of the Invention
The present invention relates to a recording medium having an air-sandwich structure, and more particularly to a recording medium of air-sandwich structure for writing and/or reading information by means of laser beam of high energy density.
2. Description of Prior Arts
Information recording mediums utilizing a beam of high energy density such as laser beam have been developed in recent years and are put to practical use. Such recording medium is called optical disc and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal such as Bi, Sn, In or Te or a semi-metal provided on said substrate. Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam and a rise in temperature locally occurs and as a result, a chemical or physical change is caused to alter optical characteristics of the recording layer in the irridated area, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alternation in the optical characteristics of the recording layer.
As a disc structure for protecting the recording layer, there has been recently proposed an air-sandwich structure in which a recording layer is provided on at least one of two disc-shaped substrates and positioned inside the two substrates opposed to each other at a given distance by interposing a ring-shaped inner spacer and a ring-shaped outer spacer between the substrates so as to form space surrounded by these two substrates and these two spacers. In the optical disc having such a structure, the recording layer is not directly exposed to the outer air and the recording and reproduction of information is conducted with laser beam passing through the substrate. Accordingly, in general there is no possibility that the recording layer is physically or chemically damaged or the surface thereof is covered with dust which is to interfere with the recording and reproduction of information.
It is known that an optical disc of the air-sandwich structure type is transported, used and stored under various environmental conditions. Under the variation of the conditions, there occurs a difference in pressure between the outside of the disc and the inside space thereof filled with an inert gas, etc., via change of temperature or atmospheric pressure dependent on altitude. As a result, the contraction (reduction in the volume) of the internally filling gas or the expansion of said gas occurs to cause deformation of the substrates. The deformation of the substrates is liable to make a failure in the operation of focus servo and further serious deformation may cause the breakage of the substrates. Particularly, the two opposed substrates are easily brought into contact with each other by the concentration (reduction in the volume) of the filling gas under low-temperature or high-pressure environmental conditions. Thus, there is caused a serious problem that the inner recording layer is damaged by the contact between both substrates. The damage of the recording layer seriously interferes with the recording and reproducing of information in a recording medium where the recording is made by changing the optical characteristics of the recording layer through a physical change such as pit formation.